For example, as illustrated in FIG. 1, a substantially tubular (bellows-shaped) bumper spring 2 formed of an elastic material, e.g., a rubber or a foamed polyurethane resin is externally fitted onto a piston rod 31 of a shock absorber 30 serving as a component of a suspension of a vehicle, and the bumper spring 2 is arranged between a cylinder (absorber plate) 32 of the shock absorber 30, and a fitting portion thereof on a vehicle body side (upper support 33) (see Patent Literature 1).
As a typical example of the bumper spring for a vehicle, there is given a urethane bumper spring using a foamed polyurethane resin (hereinafter abbreviated as “urethane”). In addition, as a method of producing the urethane bumper spring, mold-forming involving using a forming mold (die) is generally used (see Patent Literature 2).
The mold-forming of the urethane is a method of obtaining a urethane bumper spring as a product involving: injecting (casting) a urethane raw material into a forming cavity of a forming mold (die) formed of a master mold and a core, followed by heating, to thereby cure the urethane raw material through a reaction (foaming) in the forming mold; and then removing the resultant from the forming mold, followed by extraction of the forming mold core inside a tubular urethane molded body (demolding). As a foaming/curing method for the urethane, there is known a method involving two-stage heating steps, i.e., primary vulcanization and secondary vulcanization. In that case, the forming mold core inside the urethane molded body subjected to the primary vulcanization (semi-cured article) is extracted (demolding is performed) before the urethane molded body is further heated to be subjected to secondary vulcanization, and thus the urethane bumper spring is obtained as a completely cured article (product) (see Patent Literature 3).